Material Selection

Material Selection

• Getting the optimum balance of performance, quality and cost requires a careful combination of material and plastics part design

• Once end use requirements have been specified, the designer can begin searching for plastic materials that are suitable for the application

Material Selection

• Material selection/screening process is accomplished by comparing material properties with a property profile derived from the end use requirements

• Best to select 3-6 materials during the initial process based on the property profile and costs

Material Selection

• Material Selection should be based primarily on the functional requirements of the parts– Those items without which the part will not

work

• Then go to design considerations– Those items involving manufacturing factors

without which the part cannot be made profitably

Material Selection

• Functional requirements are physical properties– The ability to operate over a wide range of

temperatures

– Resistance to chemicals and environments

– Any other condition that can be described by testable parameters

• Some functional are more important than others– These should be given a top priority

Material Selection

• Design considerations are the– Raw material costs– Processing cost– Tooling costs– Recyclability– Familiarity of material– Acceptance of material in the market place

Material Selection

• Final choice of material almost always involves compromises between functional requirements and design considerations

Property Profile

Step 1

• Develop property profile

• The polymer required is classified and ranked according to the following representative grouping of functional requirements

• The list can be expanded or filtered to fit particular needs

Step 1

• Rigid

• Flexible

• Opaque

• Translucent

• Transparent

• High strength to weight

• Ordinary strength to weight

• High Temperature resistance

• Sterlizable

• Ductile at low temperatures

• Resistant to chemicals

• Dielectric constant

• Arc resistance

• Nonflammable

• Slow burning

• Very low density

• Ordinary density

• Wear resistance

• Friction coefficient

• Acceptable for food

• Post-consumer recyclable

• Weather resistant

Step 2

• It is now possible to refer material property data sheets and databases to arrive at a short list of candidates that match with the property profile developed in Step 1

• If similar application exist, begin with the current materials

Step 3

• Material properties are used to select likely candidates with corresponding property value or ranking listed

• Best to begin with properties that cannot be enhance by design– Coefficient of thermal expansion

– Transparency

– Chemical resistance

– Softening temperature

Step 3

• Then go to mechanical properties, which can generally be enhanced by design– Wall thickness– Ribs– Reinforcements

Step 4

• The next step in the elimination process is to consider items such as– Creep modulus

– Dielectric strength

– Mold shrinkage

– Costs

• The designer may want to consider statistically designed experiments to narrow the list of candidates

Material Selection Example

• There are a wide range of properties within polymer families and between families

• The problem here is to select a injection moldable polymer based on mechanical properties as a first priority

• The property profile is developed first

Material Selection Example

• Property Profile

Material Selection Example

Material Selection Example

• In the preceding table all the resins checked yes meet the preliminary specification

• The next step is to consider secondary matters such as creep, dielectric strength, shrinkage and cost.

Material Selection Example

Material Selection Example

• From the data in the preceding table, the higher cost resins can be eliminated first

• The filled resins can be eliminated next because they are more difficult to process

• This leaves 6/6 nylon and PPO• The HDTUL of nylon is must lower than PPO• The shrinkage values for PPO are much less than

nylon• PPO is the best choice